The flexible polymer market offers a wide variety of adhesive polymers, including such compounds as polyurethanes, polysulfides, silicones, and epoxy compounds. In particular, epoxy compounds demonstrate the ability to strongly adhere to a variety of materials, including metal, glass, plastic, wood, and fiber, and consequently are often used to bond dissimilar materials. Further, epoxy compounds are known to exhibit excellent resistance to attack by many corrosive chemicals. Despite their abilities to bond dissimilar materials and resist chemical attack, commercially-available epoxy compounds lack certain properties necessary for use as electrically-conductive adhesives in automated bonding processes. More particularly, an epoxy-based, electrically-conductive adhesive must also be convenient to store and readily curable to form sufficiently flexible bonds. Presently, electrically-conductive epoxy-based adhesives are available in two forms, namely two-component systems or one-component systems, neither of which is both convenient to store and readily curable.
Two-component epoxy-based adhesives are readily curable at room temperature but are inconvenient to use and store. The components of two-component systems must be accurately measured and properly mixed just prior to use. Thus, the various components to be mixed must be separately stored until use, and production workers are charged with the added responsibility of preparing epoxy-based adhesives having uniform properties. Not surprisingly, two-component epoxy-based adhesives are not favored.
One-component epoxy-based adhesives are available for industrial application in two basic forms: rigid epoxy adhesives and frozen pre-mix, flexible epoxy adhesives. While such adhesive compositions are conveniently stored as a single component, they require curing at elevated temperatures. Rigid epoxy adhesives include such compounds as Bisphenol-A epoxy adhesives and novolacs. These rigid epoxy adhesives exhibit strong adhesion for many materials and may be conveniently stored at room temperature. However, rigid epoxy-based adhesives form brittle bonds that are often insufficiently pliant for bonding dissimilar materials. For example, a brittle bond between dissimilar materials with different thermal expansion rates may be unable to withstand the stresses caused by the thermal mismatch, so that both the bond and its adherends may be susceptible to failure.
Frozen pre-mix, flexible epoxy adhesives are also employed by industry, although the usage of such adhesives is far surpassed by the usage of rigid epoxy adhesives. A description of frozen pre-mix, flexible epoxy adhesives is found in U.S. Pat. No. 4,866,108, assigned to the present assignee, which discloses and claims the composition behind Flexipoxy 100 Adhesive, a frozen flexible epoxy adhesive developed for spacecraft electronic applications. In comparison to rigid epoxy adhesives, flexible epoxy adhesives form more pliable bonds that are capable of successfully adapting to stresses between dissimilar materials caused by differing rates of expansion. However, in contrast to rigid epoxy adhesives, frozen pre-mix flexible epoxy adhesives must be stored in a frozen state and must be thawed prior to use. Moreover, frozen adhesives offer a limited working life of only about 2 to 8 hours once thawed, whereas at least one week of working life is realistically required for general automated bonding operations. Therefore, frozen pre-mix, flexible epoxy adhesives are widely considered impractical for use in high volume automated processing given the scheduling difficulties wrought by both the need to thaw the adhesives as well as the limited working life available after thawing.
Thus, a need remains for a one-component electrically-conductive epoxy adhesive that offers the convenience of room temperature storage and low temperature, rapid curing while also providing pliable bonds capable of withstanding the rigors of varying expansion rates between bonded materials. This need is particularly acute for high volume automated bonding operations, which can tolerate neither frequent periods of downtime for thawing nor the expense of discarding adhesives that quickly become ineffective. This need must be achieved without sacrificing good adhesion characteristics.